Like in the brain blood flow in the retina is coupled to neuronal activity. This phenomenon has been measured by different study groups with non invasive techniques in the brain and retina. We therefore use a Zeiss fundus camera for the assessment of retinal vessel diameters. This so called retinal vessel analyzer (RVA) is a combination of a fundus camera connected to a high resolution video camera equipped with a software based analyzing system. An unprecedented reproducibility and sensitivity of retinal vessel diameter measurements is attained with this system. In addition this system allows real time analysis of retinal vessels as well as off-line determinations from video tape. A special provocation test, which minimizes risk and discomfort to the subject under study is applied through the illumination pathway of the fundus camera: Diffuse luminance flicker is used as a stimulus to augment intrinsic mechanisms by which the retina can vary the vascular supply, in correspondence with local variations of functional activity. This system allows to study the flicker response of retinal vessels, which is within a magnitude of 6 to 8%.